Transmission with HDSL on copper pairs in the local loop is limited in range by cross talk between pairs in the same cable. To cover the range between telecom operators access points and subscriber locations, the use of transmission regenerators are necessary.
For maintenance and error detection and recovery, fault isolation on such systems must be achievable. A way of obtaining this is to provide the regenerators with a facility of looping the signal back to the access points. The regenerators are not easily accessible (dug into the ground), and thus a remote controlling of this looping is needed.
For HDSL (High speed Digital Subscriber Line) transmission systems, a regenerator management is defined as part of the transmission overhead (EOC) channel between the two endpoints of the line, NT and LT respectively. (References; TS 101135 section 5.7.5 from ETSI and G.991.1 section B5.7.1. from ITU.) The standards define the same set of functions for the regenerator as for the endpoints. FIG. 1 shows the HDSL access line.
In practical systems, the endpoint functionality is expensive, and a limited functionality is very often preferred. The use of the overhead channel for regenerator maintenance also requires that this channel is terminated and regenerated, and this introduces transmission delay and implementation complexity. A regenerator is powered via the transmission lines, and reduction of power consumption, which can be achieved by lower complexity, will extend the distance for remote powering. A pure transmission repeater without overhead channel processing should be a more optimum solution, but it requires another solution of maintenance loop setting.